Rope Too Short? What Can You Do?

To point 1 -

A knot won't pass through an autoblock...

What are the circumstances where a knot comes up on your rappel grip and you proceed to assist it in feeding through your figure 8 device?

To point 2 -

It's all good until it isn't. If one doesn't tie off below the ascenders and something goes wrong, is the result worth the risk?

Great discussion! I'm pretty late to the party, but I'll still post my experience.
A few years ago when I was younger and less wise, I came up 10 ft short on a waterfall rappel. Online beta indicated the drop was 35 ft, so I thought 100ft doubled roped would be plenty, but it was actually a 60 ft drop. I couldn't tell from the top that I was short, because the waterfall blocked my view. About half way down the rap, I could see that the rope was somewhat short. There wasn't anybody up top, but a hiker standing at the bottom said I was 10 ft short. Being over six feet tall having a reach of about eight feet, I reasoned that I could hang from the end of the rope and only have to drop two or three feet from boots to bottom. So I carefully rapped off the ends of the double strands, but they were too wet and slippery to hang onto. I fell and bounced off a slick wall behind the waterfall, and landed in the pool at the bottom, which was about 18 inches deep. The impact forced me flat on the bottom of the pool, and as I laid there the waterfall was blasting me in the head - but I was OK!! I learned from that experience!